Salicylic acid regulates sugar metabolism that confers tolerance to salinity stress in cucumber seedlings

Salicylic acid (SA) plays a critical role in plant development and defense responses to biotic and abiotic stresses. Sugars can act as osmotic adjustors or nutrient and metabolic signals in the activation of plant defense responses. To uncover the effects of SA on sugar metabolism, we assayed the altered components and levels of sugars in cucumber seedlings treated with SA. After SA treatment, the levels of glucose, fructose, raffinose and stachyose were increased in both leaves and roots. In contrast, sucrose and starch responded differently in leaves and root, decreasing in leaves but accumulating in roots. These changes could be due to the effects of SA on the activities of metabolism-related enzymes. In leaves, SA promoted the activities of sucrose phosphate synthase (SPS), sucrose synthase (SS) and amylases, while the SA-treated root showed a reduced amylase activity and an unchanged activity in SPS. The changes in various sugar contents resulted in the accumulation of soluble sugars in SA-treated cucumber seedlings, especially non-reducing sugars in roots. These increased sugars could function as osmotic regulators and facilitate water uptake and retention in plant cells, thereby conferring seedlings an enhanced tolerance to salinity stresses caused by NaCl treatment. Taken together, our findings provide an important insight into the effects of SA on sugar metabolism, and a protective mechanism for SA against water deficiency is discussed. (C) 2011 Elsevier B.V. All rights reserved.